But this doesn't invalidate the OP's point... regardless of how reasonable you believe a change in speed limit to be, you're still obligated to follow it, and if you don't then you might get a ticket. And it's not like these changes are unannounced; there is a sign for every time it changes. Surely if you're going to listen to some "speed trap detecting" contraption, you could also look at the signs that announce the change in speed limit? Or better yet, whatever app you're using to detect speed cameras should give you an equivalent ping every time it detects a change in speed limit...